1. Field of the Invention
The present invention relates to a photoelectric conversion device and an imaging device.
2. Description of the Related Art
As for the solid-state imaging device, there is widely used a flat light-receiving device where photoelectric conversion sites are two-dimensionally arrayed in a semiconductor to form pixels and a signal generated by photoelectric conversion in each pixel is charge-transferred and read out according to a CCD or CMOS format. The conventional photoelectric conversion site is generally formed, for example, by forming PN junction in a semiconductor such as Si.
In recent years, with the progress of a multi-pixel system, the pixel size and in turn, the area of a photodiode part becomes small, and this brings about problems of reduction in the aperture ratio and reduction in the light gathering efficiency. As for the measure to enhance the aperture ratio and the like, studies are being made on a solid-state imaging device having an organic photoelectric conversion film using an organic material.
A technique of introducing a bulk heterostructure using a fullerene or a fullerene derivative into the organic photoelectric conversion film so as to bring out high photoelectric conversion efficiency (high exciton dissociation efficiency) is known.
For example, Patent Document 1 discloses a photoelectric conversion film containing a fullerene or a fullerene derivative.
The organic photoelectric conversion device used in a solar cell is designed to collect electric power and therefore, an external electric field is not applied, but the photoelectric conversion device used as a visible light sensor of a solid-state imaging device needs to maximize the photoelectric conversion efficiency and a voltage is sometimes externally applied so as to enhance the photoelectric conversion efficiency or increase the response speed.
When a voltage is externally applied so as to enhance the photoelectric conversion efficiency or increase the response speed, injection of a hole or injection of an electron from an electrode is generated due to an external electric field, and this disadvantageously increases the dark current.
Many of materials usually used as an electrode in a photoelectric conversion device have a work function (WF) of around 4.5 eV (for example, ITO) and, for example, in the case of using a fullerene as the material of the photoelectric conversion film, an energy gap between the WF of electrode and the LUMO of fullerene (C60) becomes small, as a result, particularly an electron is liable to be injected from the electrode into the photoelectric conversion film and a significant increase of the dark current is caused.
As regards the prevention of an increase in the dark current due to an injected current, a technique of providing a charge blocking layer to suppress the injection of an electric charge into the photoelectric conversion layer, thereby efficiently preventing an injected carrier and reducing the dark current, is disclosed (Patent Document 2).
Also, in Patent Documents 3 and 4, an organic material having a carbazole structure is disclosed, but its use as a charge blocking layer or the like of a photoelectric conversion device is not described at all.    [Patent Document 1] JP-A-2007-123707 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)    [Patent Document 2] JP-A-2008-72090    [Patent Document 3] JP-A-2005-290000    [Patent Document 4] U.S. Pat. No. 6,649,772